The various techniques used to perform percutaneous guided biopsies of mediastinal masses have already been amply described, with particular attention to the type of needle used. In this study our experience with mediastinal transthoracic core needle biopsy has been compared with reported descriptions of ultrasonically guided fine needle aspiration to determine which is better and the respective influence on pathological diagnosis. Between January 1998 and July 2002, 47 patients underwent anterior mediastinal core needle biopsy with ultrasonic guidance. An accurate diagnosis was achieved in all patients, with 100% sensitivity and specificity. In all patients with lymphoma and thymoma it proved possible to establish the histological type. Two cases of pneumothorax were seen with pleural drainage and a 5-day hospital stay required in one of the cases. The remaining 45 patients were treated as outpatients and were discharged within 4 hours of the procedure. Ultrasonically guided percutaneous needle biopsy is a safe procedure; fine needle aspiration usually suffices for solid malignant lesions while a core needle biopsy should be performed when lymphoma or thymoma masses are suspected in order to obtain larger specimens for a precise histological diagnosis. The core needle procedure avoids repetition of unsuccessful fine- needle aspirations and reduces the number of mediastinoscopies and videothoracoscopies carried out for diagnostic purposes.